Having muddled through the process of creating a private host overlay
with much help from #chromium-os, document the details and pitfalls.

BUG=None
TEST=Read the new documentation and wished I'd had it earlier this week.

Change-Id: Ia26d64324f1ffb7702973846c2b5e765ba5a2c2a
Reviewed-on: https://gerrit.chromium.org/gerrit/34834


Tested-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>